Apparatus for plasma dicing

ABSTRACT

An apparatus is for plasma dicing a semiconductor substrate of the type forming part of a workpiece, the workpiece further including a carrier sheet on a frame member, where the carrier sheet carries the semiconductor substrate. The apparatus includes a chamber, a plasma production device configured to produce a plasma within the chamber suitable for dicing the semiconductor substrate, a workpiece support located in the chamber for supporting the workpiece through contact with the carrier sheet, and a frame cover element configured to, in use, contact the frame member thereby clamping the carrier sheet against an auxiliary element disposed in the chamber.

This is a Continuation of U.S. application Ser. No. 15/293,153, filedOct. 13, 2016, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND

This invention relates to apparatus for plasma dicing a semiconductorsubstrate of the type forming part of a workpiece, the workpiece furthercomprising a carrier sheet on a frame member, wherein the carrier sheetcarries the semiconductor substrate. The invention relates also toassociated methods of plasma dicing.

Semiconductor manufacture typically involves large numbers of siliconchips being processed in parallel on a single semiconductor wafer. Oncethe processing steps are completed, the wafer must be diced intodiscrete chips. Connections are then made to the chips which aresubsequently packaged. Traditionally, the dicing step has been carriedout using diamond saws which cut along scribe lines on the wafer.Recently, there has been much interest in providing new methods fordicing wafers. These offer the possibility of enhanced performance andlower cost. One such approach is to use lasers to carry out the dicingof the wafer. An alternative approach is to use plasma etching to dicethe wafer. This has benefits in terms of minimising die edge damage,maximising the efficient use of the wafer surface by using very narrowscribe lines, and providing the option to use non-orthogonal scribe linelayouts. Depending on the application requirement, the plasma etchingcan occur before or after thinning or grinding of the wafer.

Silicon wafers are presented for dicing on a “frame and tape” carrier.FIG. 7 is a semi-schematic representation of such an arrangement showinga silicon wafer 71 with discrete chips 73 separated by scribe lines 72.The wafer 71 is adhered by an adhesive to a carrier tape 74 which isretained by an annular frame ring 76. A plasma 9 is used to etch theexposed scribe lines in a vacuum system (not shown). The wafer71/carrier tape 74/frame 76 forms a workpiece assembly which is placedon a platen 75. The platen retains the workpiece, provides cooling, andoptionally an RF bias to aid the etch process. Frequently, anelectrostatic chuck (ESC) is used to improve heat coupling between thewafer and the platen. U.S. Pat. No. 8,691,702 discloses an ESC basedplaten assembly which can process taped wafers on a frame. A liftmechanism is provided which raises the workpiece to allow it to be movedto and from the platen. A frame cover spaced apart from the frame isprovided which protects the lift mechanism and the frame. An extensionof the frame cover or a separate component is provided to protect thetape in the vicinity of the frame. The ESC is provided with appropriatecooling channels to remove heat from the chuck. The removal of heatduring the process in order to maintain suitable working temperatures isan important consideration. The carrier tape is at particular risk ofoverheating. Typically, the carrier tape is formed from a polymericmaterial such as a polyolefin (PO) polyvinyl chloride (PVC) orpolyethylene terephthalate (PET) with a softening point of around 90° C.In practice, it is considered necessary to maintain the temperature ofthe carrier tape at 80° C. or less during processing to avoid damage tothe tape or to the acrylic adhesive used on the tape. Thermal runaway isa constant risk as the thickness of the tape is typically less than 200microns and therefore the tape has a low thermal capacity.

It is desirable for economic reasons to dice wafers as quickly aspossible while maintaining the process specification. This is typicallyachieved by running the plasma etch tool at a high RF power to increasethe exposed material in the scribe lines of the semiconductor wafer. Theuse of high etch rate regimes increases the likelihood of overheating.Therefore, there is a strong desire to achieve high etch rates in plasmadicing to maximise throughput whilst avoiding damage to the workpiece,in particular avoiding damage to the carrier tape or the associatedadhesive. It is particularly desirable to be able to maintain thetemperature of the carrier tape at 80° C. or less.

SUMMARY

The present invention, in at least some of its embodiments, addressesthe above described problems and wants.

According to the first aspect of the invention there is provided anapparatus for plasma dicing a semiconductor substrate of the typeforming part of a workpiece, the workpiece further comprising a carriersheet on a frame member, wherein the carrier sheet carries thesemiconductor substrate,

the apparatus comprising:

a chamber;

a plasma production device configured to produce a plasma within thechamber suitable for dicing the semiconductor substrate;

a workpiece support located in the chamber for supporting the workpiecethrough contact with the carrier sheet;

a frame cover element configured to, in use, contact the frame memberthereby clamping the carrier sheet against an auxiliary element disposedin the chamber.

The auxiliary element may be the workpiece support.

The auxiliary element may be a shield ring which is disposed around theworkpiece support. The shield ring may be in thermal contact with thechamber. Typically, the shield ring may be in thermal contact with oneor more interior walls of the chamber.

The apparatus may further comprise a heat shield disposed over the framecover element to thermally shield the frame cover element from theplasma.

The heat shield may be spaced apart from the frame cover element. Theheat shield may be supported on the auxiliary element.

The heat shield may be in contact with the frame cover element.Typically, in these embodiments, it is desirable to minimise the thermalcontact between the heat shield and the frame cover element. The heatshield may comprise one or more protrusions which contact the framecover element. In this way, thermal contact can be reduced. Theprotrusions may be of any suitable form, such as pins, one or moreridges, or pips.

The heat shield may be formed from a ceramic material. The ceramicmaterial may be alumina.

In the other main embodiments, there is no heat shield present tothermally shield the frame cover element from the plasma.

The apparatus may further comprise at least one clamp which applies aclamping force to the frame cover element to assist the clamping of thecarrier sheet against the auxiliary element by the frame cover element.The at least one clamp may directly clamp the frame cover element, or ifthe heat shield is present, the at least one clamp may clamp the heatshield against the frame cover element to assist the clamping of thecarrier sheet against the auxiliary element. The at least one clamp maybe mounted on the workpiece support.

The frame cover element may comprise a substantially flat lower surfacewhich, in use, contacts the frame member.

The frame cover element may comprise one or more protrusions which, inuse, contacts the frame member. The protrusions may be of any suitableform, such as pins, one or more ridges, or pips. The auxiliary elementmay comprise one or more openings for receiving the protrusions of theframe cover element when a workpiece is not present on the workpiecesupport.

The apparatus may be configured so that the frame cover element can bebrought into thermal contact with the auxiliary element when a workpieceis not present on the workpiece support.

The frame cover element may be formed from a metal or a ceramicmaterial. An example of a suitable metal is aluminium. An example of asuitable ceramic material is alumina.

The workpiece support may be an electrostatic chuck (ESC). The ESC maybe a monopolar or a bipolar ESC.

The apparatus may further comprise a lifting mechanism for lowering andraising the frame into and out of contact with the workpiece support andlowering and raising the frame cover element into out of contact withthe frame member and, optionally, the workpiece support.

The apparatus may be provided in combination with a workpiece comprisingthe semiconductor substrate and a carrier sheet on a frame member,wherein the carrier sheet supports the semiconductor substrate, theworkpiece support supports the workpiece through contact with thecarrier sheet, and the frame member is clamped by the frame coverelement.

The workpiece support may comprise a workpiece support surface whichsupports the workpiece through contact with the carrier sheet. Theworkpiece support surface has a periphery. The frame member may define aperiphery of the workpiece. The periphery of the workpiece may be whollylocated within the periphery of the workpiece support surface.

The carrier sheet may comprise tape formed from a polymeric material,optionally with an adhesive. The polymeric material may be a PO, PVC orPET.

According to a second aspect of the invention there is provided a methodof plasma dicing a semiconductor substrate of the type forming part of aworkpiece, the workpiece further comprising a carrier sheet on a framemember, wherein the carrier sheet carries the semiconductor substrate,the method comprising the steps of:

providing an apparatus according to the first aspect of the invention;

supporting the workpiece by placing the carrier sheet in contact withthe workpiece support;

clamping the carrier sheet against the auxiliary element by contactingthe frame member with the frame cover element; and

plasma dicing the semiconductor substrate.

The workpiece support may comprise a workpiece support surface whichsupports the workpiece through contact with the carrier sheet. Theworkpiece support surface has a periphery. The frame member may define aperiphery of the workpiece. The periphery of the workpiece may be whollylocated within the periphery of the workpiece support surface.

The workpiece support may be an electrostatic chuck and the portion ofthe electrostatic chuck which contacts the carrier film may be asubstantially flat and featureless upper surface.

The workpiece support may be an electrostatic chuck which acts as theauxiliary element, wherein the electrostatic chuck provides anadditional, electrostatic clamping force acting on the frame member.

After the step of plasma dicing the semiconductor substrate iscompleted, the workpiece may be removed from the chamber and the framecover element may be brought into thermal contact with the workpiecesupport.

Whilst the invention has been described above, it extends to anyinventive combination of the features set out above, or in the followingdescription, drawings or claims. For example, any feature described inrelation to the first aspect of the invention is considered to bedisclosed also in relation to the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of apparatus and methods in accordance with the inventionwill now be described with reference to the accompanying drawings, inwhich:

FIG. 1 is a cross sectional view of semiconductor substrate mounted invacuum processing chamber with additional heat shield;

FIG. 2 is a cross sectional view of vacuum processing chamber (fromFIG. 1) during a cooling stage, which occurs in between successiveplasma treatments;

FIG. 3 is a cross sectional view of a semiconductor substrate mounted ina vacuum processing chamber where clamping pins are used to clamp theannular frame to the dicing tape;

FIG. 4 is a cross sectional view of the vacuum processing chamber (fromFIG. 3) during a cooling stage which occurs between successive plasmatreatments, where clamping pins are lowered into recesses in the ESC;

FIG. 5 is a cross sectional view of the heat shield optimised design inits lowered position;

FIG. 6 is a cross sectional view of the heat shield optimised designs inits raised position; and

FIG. 7 shows plasma dicing of a workpiece comprising a wafer supportedon a tape and frame carrier.

DETAILED DESCRIPTION OF EMBODIMENTS

A first embodiment of the invention is shown in FIG. 1. A semiconductorsubstrate 11 comprises scribe lines 12 and discrete semiconductor chips13. The semiconductor substrate 11 is typically composed of silicon,however, gallium arsenide and other III-V semiconductors may be used.The semiconductor substrate 11 is adhered to the dicing tape 14, whichis positioned on top of an electrostatic chuck 15. An annular frame 16is positioned on top of the dicing tape 14 such that the dicing tape 14is fixed between the annular frame 16 and the electrostatic chuck 15.The semiconductor substrate 11 is designed to accommodate a degree ofnon-concentricity, typically ±3 mm, in the positioning of the annularframe 16. The frame assembly 17 comprises the semiconductor substrate11, the dicing tape 14, and the annular frame 16. The dicing tape 14 istypically composed of polyolefin, poly(vinyl chloride), or poly(ethyleneterephthalate). The annular frame 16 is typically composed of stainlesssteel or plastic. The surface area of the frame assembly 17 and theelectrostatic chuck 15 are selected so that the electrostatic chuck 15extends beyond the diameter of the annular frame 16 and containsinternal cooling channels 18 where a coolant gas is passed. A highvoltage may be applied to the electrostatic chuck 15 to provide a firstclamping force to the frame assembly 17. The electrostatic clampingmechanism enables a good thermal contact to exist between the frameassembly 17 and the electrostatic chuck 15. Furthermore, theelectrostatic chuck 15 does not contain any surface features within thediameter of the annular frame 16 in order to maximise the thermalcontact between the frame assembly 17 and the electrostatic chuck 15. Agood thermal contact between the frame assembly 17 and the electrostaticchuck 15 helps to keep the frame assembly 17 cool during plasmatreatment and prevent thermal degradation of the dicing tape 14. Theannular frame 16 is shielded from direct exposure to the plasma 19 byuse of a frame cover 110.

In one embodiment of the invention the frame cover 110 makes a goodthermal contact with the annular frame 16. The frame cover 110 may beprovided additional protection from the plasma 19 by the use of a heatshield 111. If a heat shield 111 is employed, the frame cover 110 iscomposed of a material with a high thermal conductivity, such asaluminium. However, if a heat shield 111 is not used, the frame cover110 will be in direct contact with the plasma 19 and a material with alow thermal conductivity, such as alumina or other ceramic material, ispreferred. The heat shield 111 is typically composed of a material withpoor thermal conductivity, such as alumina or other ceramic material.The contact 112 between the heat shield 111 and the frame cover 110 isminimal or non-existent to minimise the thermal pathway between the heatshield 111 and the frame cover 110. Consequently, the plasma 19 directlyheats the heat shield 111 but that heat does not permeate towards theframe assembly 17. The weight of the annular frame 16, the frame cover110, and the heat shield 111 provides a second clamping mechanism of thedicing tape 14 to the electrostatic chuck 15. The second clampingmechanism may also be achieved using an active clamping force. Theactive clamping force may be achieved through use of a platenmounted—weighted clamp or other clamping device. The active clampingforce may be applied directly to the heat shield 111, the frame cover110, the annular frame 16 or a combination thereof in order to achievethe second clamping mechanism of the dicing tape 14 to the electrostaticchuck 15.The frame assembly 17 is removed from the vacuum processingchamber 113 after the plasma treatment is complete and a cooling stageis initiated as shown in FIG. 2. The frame cover 110 is cooled prior tothe commencement of a successive plasma treatment. The cooling effect isachieved by lowering the frame cover 110 so that there is a largecontact area between the frame cover 110 and the electrostatic chuck 15.The frame cover 110 is composed of a material with a high thermalconductivity, such as aluminium, and forms a good thermal contact withthe electrostatic chuck 15. The good thermal contact between the framecover 110 and the electrostatic chuck 15 allows the heat from the framecover 110 to be dissipated easily and allow efficient cooling of theframe cover 110.

In a second embodiment of the invention, the frame assembly 17 ismechanically clamped from above to the electrostatic chuck 35 by the useof clamping pins 30 as shown in FIG. 3. The clamping pins 30 areprotrusions from the underside of the frame cover 31 and form a poorthermal contact between the frame cover 31 and the annular frame 16. Theframe cover 31 may be provided with additional protection from theplasma 19 in the form of a heat shield 111. If a heat shield 111 isemployed, the frame cover 31 is composed of a material with a highthermal conductivity, such as aluminium. However, if a heat shield 111is not used, the frame cover 31 will be in direct contact with theplasma 19 and a material with a low thermal conductivity, such asalumina or other ceramic material, is preferred.

The frame assembly 17 is removed from the vacuum processing chamber 113after the plasma treatment is complete and a cooling stage is initiatedas shown in FIG. 4. The frame cover 31 is cooled prior to thecommencement of a successive plasma treatment. The cooling effect isachieved by lowering the frame cover 31 so that it is in direct contactwith the thermally regulated electrostatic chuck 35. In this embodimentof the invention the clamping pins 30 insert into complementary recesses32 located in the electrostatic chuck 35. This ensures a high contactarea and good thermal contact between the frame cover 31 and theelectrostatic chuck 35 to allow efficient cooling of the frame cover 31during the cooling stage.

In a third embodiment of the invention, the annular frame 16 ispositioned directly on top of a shield ring 50. The shield ring 50 is anannular component surrounding the electrostatic chuck 55. Thermalprotection from the plasma 19 is provided by the use of a frame cover 51and a heat shield 52 as shown in FIG. 5. The shield ring 50 is typicallycomposed of a material with a high thermal conductivity, such asaluminium, and is kept in contact with the walls of the vacuumprocessing chamber 53. The walls of the vacuum processing chamber 53 aremaintained at a suitable temperature, typically about 55° C. The heatshield 52 provides thermal protection for the frame cover 51. The heatshield 52 rests solely on the shield ring 50 and does not contact theframe cover 51 during the plasma treatment. The heat shield 52 is incontact with the shield ring 50 via a series of equidistantly dispersedprotrusions such as pips 54. The heat shield 52 may comprise anextension in the form of a tape cover 56 to give protection to thedicing tape 14. The tape cover is typically composed of a material witha poor thermal conductivity, such as alumina or other ceramic material.The heat shield 52 and tape cover 56 are in direct line of sight to theplasma 19 and may reach temperatures in excess of 150° C. The heatshield 52 and tape cover 56 absorb the thermal radiation from the plasma19 but heat is not readily transferred to the shield ring 50 due to thepoor thermal contact of the protrusions 54 to the shield ring 50. Thishelps to maintain the heat shield 52 and tape cover 56 at a hightemperature. The heat shield 52 and tape cover 56 form a thermal shieldwhich results in the frame cover 51 being cooler than in prior artdesigns. The relatively cool frame cover 51 then also forms a thermalshield around the frame 16. This protects the frame 16 from radiativeand convective heat transfer from the heat shield 52 and tape cover 56.Advantageously, the high temperature of the heat shield 52 and tapecover 56 prevents the accumulation of particular deposits, whichincreases the useable lifetime in between cleaning procedures. The framecover 51 makes contact with the annular frame 16 via a series ofequidistantly dispersed protrusions such as pips 57 which act tomechanically clamp the annular frame 16 to the shield ring 50. Themechanical clamping force may be as a result of the weight of the framecover 51 or an additional clamping force may be employed. The contactarea between the protrusions 57 and the annular frame 16 is minimalwhich results in a poor thermal contact. The combination of the framecover 51 being relatively cool, and the poor thermal contact between theprotrusions 57 and the frame 16 means that plastic frames can be usedwithout substantial risk of damage. The use of metal frames is possiblealso.

In order to remove the frame assembly 17 from the vacuum processingchamber 113 the annular frame 16 and the frame cover 51 are raised usinglifting pins 60. FIG. 6 shows a cross-sectional view of the vacuumprocessing chamber with lifting pins 60 in the raised position. A poorthermal contact between the heat shield 52 and the frame cover 51 ismaintained from the use of equidistantly dispersed protrusions such aspips 61. The minimal contact area of the protrusions 61 minimises heatdissipation from the heat shield 52 to the frame cover 51. Each set ofprotrusions 54, and 57 has a corresponding recess for them to insertinto, which allows the frame cover 51, the heat shield 52 and the shieldring 50 to self-align when the lifting pins are utilised.

What is claimed is:
 1. Apparatus for plasma dicing a semiconductorsubstrate of the type forming part of a workpiece, the workpiece furthercomprising a carrier sheet on a frame member, wherein the carrier sheetcarries the semiconductor substrate, the apparatus comprising: achamber; a plasma production device configured to produce a plasmawithin the chamber suitable for dicing the semiconductor substrate; aworkpiece support located in the chamber for supporting the workpiecethrough contact with the carrier sheet; a frame cover element configuredto, in use, contact the frame member thereby clamping the carrier sheetagainst an auxiliary element disposed in the chamber.
 2. An apparatusaccording to claim 1 in which the auxiliary element is the workpiecesupport.
 3. An apparatus according to claim 1 in which the auxiliaryelement is a shield ring which is disposed around the workpiece support.4. An apparatus according to claim 3 in which the shield ring is inthermal contact with the chamber.
 5. An apparatus according to claim 1further comprising a heat shield disposed over the frame cover elementto thermally shield the frame cover element from the plasma.
 6. Anapparatus according to claim 5 in which the heat shield is spaced apartfrom the frame cover element.
 7. An apparatus according to claim 6 inwhich the heat shield is supported on the auxiliary element.
 8. Anapparatus according to claim 5 in which the heat shield is in contactwith the frame cover element, and optionally comprises one or moreprotrusions which contact the frame cover element.
 9. An apparatusaccording to claim 5 in which the heat shield is formed from a ceramicmaterial.
 10. An apparatus according to claim 1 further comprising atleast one clamp which applies a clamping force to the frame coverelement to assist the clamping of the carrier sheet against theauxiliary element by the frame cover element.
 11. Apparatus according toclaim 1 in which the frame cover element comprises one or moreprotrusions which, in use, contact the frame member, and optionally theauxiliary element comprises one or more openings for receiving theprotrusions of the frame cover element when a workpiece is not presenton the workpiece support.
 12. An apparatus according to claim 1configured so that the frame cover element can be brought into thermalcontact with the auxiliary element when a workpiece is not present onthe workpiece support.
 13. Apparatus according to claim 1 in which theframe cover element is formed from metal or a ceramic material. 14.Apparatus according to claim 1 in which the workpiece support is anelectrostatic chuck.
 15. Apparatus according to claim 1 furthercomprising a lifting mechanism for lowering and raising the frame intoand out of contact with the workpiece support and lowering and raisingthe frame cover element into and out of contact with the frame memberand, optionally, the workpiece support.
 16. Apparatus according to claim1 in combination with a workpiece comprising the semiconductor substrateand a carrier sheet on a frame member, wherein the carrier sheetsupports the semiconductor substrate, the workpiece support supports theworkpiece through contact with the carrier sheet, and the frame memberis clamped by the frame cover element.
 17. Apparatus according to claim16 in which the carrier sheet comprises tape formed from a polymericmaterial, optionally with an adhesive.
 18. A method of plasma dicing asemiconductor substrate of the type forming part of a workpiece, theworkpiece further comprising a carrier sheet on a frame member, whereinthe carrier sheet carries the semiconductor substrate, the methodcomprising the steps of: providing an apparatus according to claim 1;supporting the workpiece by placing the carrier sheet in contact withthe workpiece support; clamping the carrier sheet against the auxiliaryelement by contacting the frame member with the frame cover element;plasma dicing the semiconductor substrate.
 19. A method according toclaim 18 in which: the workpiece support comprises a workpiece supportsurface which supports the workpiece through contact with the carriersheet and which has a periphery; the frame member defines a periphery ofthe workpiece; and the periphery of the workpiece is wholly locatedwithin the periphery of the workpiece support surface.
 20. A methodaccording to claim 18 in which the workpiece support is an electrostaticchuck and the portion of the electrostatic chuck which contacts thecarrier film is a substantially flat and featureless upper surface.